Non-volatile memory devices are used in certain applications where data must be retained when power is disconnected. Applications include general memory cards, consumer electronics (e.g., digital camera memory), automotive (e.g., electronic odometers), and industrial applications (e.g., electronic valve parameter storage). The non-volatile memories may use phase-change memory materials, i.e., materials that can be switched between a generally amorphous and a generally crystalline state, for electronic memory applications. The memory of such devices typically comprises an array of memory elements, each element defining a discrete memory location and having a volume of phase-change memory material associated with it. The structure of each memory element typically comprises a phase-change material, one or more electrodes, and one or more insulators.
One type of memory element originally developed by Energy Conversion Devices, Inc. utilizes a phase-change material that can be, in one application, switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. These different structured states have different values of resistivity and therefore, each state can be determined by electrical sensing. Typical materials suitable for such applications include those utilizing various chalcogenide materials. Unlike certain known devices, these electrical memory devices typically do not use field-effect transistor devices as the memory storage element. Rather, they comprise in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little area is required to store a bit of information, thereby providing for inherently high-density memory chips.
The state change materials are also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until reprogrammed, as that value represents a physical state of the material (e.g., crystalline or amorphous). Further, reprogramming requires little energy to be provided and dissipated in the device. Thus, phase-change memory materials represent a significant improvement in non-volatile memory technology.
However, present phase-change memories may include losses in heat and may require large programming volumes of the phase-change material. The heat losses may be due to heat transfer to adjacent memories (in the context of a memory array) or to adjacent structures such as electrodes and interconnects. Large programming volumes may be attributed to electrode structures that spread current over a larger area, rather than concentrating the current to a smaller region. Additionally, present devices experience degradation over time due to atomic migration between the dissimilar materials of the phase-change material and the electrode material.
Therefore, a need has arisen to produce an improved phase-change memory device that may provide for reduced programming currents. It is also desirable to reduce memory degradation over time.